In the exploitation of under sea hydrocarbon reserves, production platforms have usually been fixed structures which were piled to or rest firmly on the seabed. Fixed structures are very expensive in deep water, too expensive for small shallow water fields with short productive lives, and are difficult to remove once a field is depleted. Accordingly, floating platforms have been developed and two fields producing hydrocarbons in September 1980 have floating production systems installed. The first such system began producing oil from the Argyll field in 1975 and uses a semi-submersible rig, "Transworld 58", to process oil flowing from four scattered appraisal wells. Flow lines are connected to a single manifold under the rig and continue upwards through individual lines of a composite flow line or riser. The second such floating system began producing oil in 1980, and uses a semi-submersible rig, "Sedco I" to process oil flowing from three wells drilled on the seabed directionally from a cluster directly beneath the rig. Each of the three wells has an individual hydrocarbon flowline or riser from its subsea master valve block to the platform. In the platform there is an area termed the "moonpool" area containing the stations where the risers are received in the platform. An arrangement such as this latter arrangement offers the advantages of relatively vertical flow paths for fluids produced from subsea wells, simpler subsea equipment, and direct vertical access for maintenance/repair work. For these reasons, this provides the preferred method for multi-well floating hydrocarbon production systems in water depths to at least 300 meters. The "moonpool" is constituted by an opening through at least the main deck area of the platform and, in conventional construction, is provided with a roof constituted by a platform top deck which carries the derrick. It is normally the case that the area above the moonpool under the platform top deck or moonpool roof is obstructed by cables and other equipment. This present invention is based upon the appreciated that the freeing of this space and its occupation by a moveable crane provides substantial advantages in floating platform maintenance and operation.
Examples of multi-well floating production platform designs having individual hydrocarbon flowlines/risers from each well to the platform are the PRODUCAT design by Forex Neptune in the magazine Ocean Industry, October 1977, pages 53 to 56, and the Tension Leg Platform, described in Ocean Industry, February 1980, pages 35 to 39 and in paper No. 3881 presented at the Offshore Technology Conference in Houston, Tex., U.S.A. in May 1980. Floating platforms, whether anchored to the seabed by catenary or vertical lines, or dynamically positioned, are subject to marine motions which prevent the rigid support of fluid risers from the seabed. So-called riser tensioners or riser tensioning systems have therefore been developed.
Subsea wells drilled into hydrocarbon bearing formations are usually lined with a cemented steel casing and fluid produced from the well rises up concentric tubing which has a sealing packer at the lower end thereof. The tubing string may have a full-bore down-hole safety valve screwed into it, and the resulting assembly may be suspended from a tubing hanger in the subsea wellhead. A subsea valve block (sometimes called a "Christmas tree") stabs into the tubing hanger and connects to the wellhead. A flowline or riser leads from the subsea valve block to the surface and permits the conveyance of fluids produced to the process equipment. If a problem arises with the well equipment below the subsea valve block, a maintenance operation called "major workover" is required on the subsea well. This requires the use of a maintenance line or "workover riser" and for precisely the same reasons that tension needs to be applied to the individual well risers, tension must also be applied to the workover riser. Examples of problems which may be encountered requiring the "major workover" operation include tubing corrosion, sliding sleeve valve or tubing-retrievable safety-valve failure, or packer leakage. Access to the well below the subsea valve block in order to perform "major workover" is only possible from a floating production platform if a structure termed a blow-out preventer (BOP) stack is built into the floating platform in addition to the workover riser, a riser tensioning system and "kill" fluid. The BoP stack is a known assembly designed to provide control over fluid flow into and/or out of a well during drilling or workover operations when no other mechanical means (such as valves) are available.
A floating platform may, for example, have ten risers leading from corresponding subsea wells each received in the moonpool area. It may, of course, be necessary to perform "major workover" on any one of these wells but it has been found that small tensioners of the type appropriate for ordinary well flowines/risers (production risers) are inadequate to support a workover riser properly. Typical production risers may be about seven inches in diameter and the appropriate riser tensioners exhibit up to 60,000 lbs pull. In contrast, the workover riser may have a minimum diameter of 16 inches with external choke and "kill" lines and may need a minimum 320,000 lbs of tension capacity to be available for safety. Thus, there is a need for tensioning systems for such workover risers.